A technique has been known in the past, according to which a laser beam is projected to an object to be measured, and a reflection light from the object to be measured is received by a photodetection element, and a distance to the object to be measured is determined.
In general, it is known in electro-optical distance measurement that error may occur in a result of distance measurement due to light quantity of a reflection light from the object to be measured. This is because electrical phase characteristics of a photodetection element and a photodetection circuit are subject to slight and minute change when a receiving light quantity is changed. For the purpose of reducing the changes of the phase characteristics as much as possible, it has been practiced in the past to perform an electric adjustment on the photodetection circuit for each machine or device in the manufacturing process.
FIG. 19 is a diagram to show phase characteristics A of a photodetection circuit under non-adjustment conditions and phase characteristics B after the adjustment. In FIG. 19, the light quantity is represented on the axis of abscissa, and the phase is represented on the axis of ordinate.
Here, the term “phase” means a time T1 as shown in FIG. 20(A). The time T1 means a time period up to the moment when the photodetection signal is turned to 0 for the first time when a light emission timing signal is issued and the photodetection element outputs a photodetection signal. The time T1 means a moment when it is judged that the photodetection element receives the light.
As shown in FIG. 20(B), when receiving light quantity (received light quantity) increases, a time T2 is extended by ΔT with respect to the time T1 of FIG. 20(A), and a delay (deviation of the phase) occurs in the light receiving time.
Therefore, as seen in FIG. 19, in case of non-adjustment (phase characteristics A), it is seen that the phase is changed considerably when the light quantity is changed. For this reason, in case of non-adjustment, error may be included in the result of distance measurement.
In case of the phase characteristics B, even when the receiving light quantity is changed as the result of adjustment of the photodetection circuit, almost no change occurs in the phase, and stable phase characteristics can be maintained for wider light quantity range.
Another adjustment method is disclosed in JP-A-2004-264116 (the Patent Document 1). According to the method described in JP-A-2004-264116 (the Patent Document 1), the change of the receiving light quantity occurs over total light receiving (photodetection) range in the manufacturing process. Then, a relation between the receiving light quantity over total photodetection range and electrical phase change is determined. This relation between the receiving light quantity and electrical phase change (i.e. information of receiving light quantity—phase change) is stored in a storage unit inside the device, and error is corrected according to (information of receiving light quantity—phase change) thus stored at the time of distance measurement.
Further, the phase characteristics of the photodetection element and the photodetection circuit are also changed according to the environment where the device is used and also to the changes over time. For instance, the distance measuring device is also used in various regions on the earth including cold climate region such as Russia or in tropical forests in equatorial region. Therefore, the distance measuring device is expected so that the operation can be performed over the temperature range from −30° C. to +60° C. In such case, the temperature of the photodetection element or the photodetection circuit is changed. As a result, changes also occur in the relation between the receiving light quantity and electrical phase characteristics, and the changes often appear as measurement error.
Because the error caused by the change of environment cannot be eliminated by electrical adjustment of the photodetection circuit at the time of manufacture, the photodetection circuit must be composed of electrical components with high performance characteristics over wide temperature range so that measurement error may be within the change specified in the specification of the product even when environmental conditions have changed. Accordingly, higher costs are required for the components and the manufacturing cost is increased.
When many years have elapsed after the manufacture, electrical characteristics at the time of manufacture are changed due to deterioration of the quality of electronic components. In this case, deviation gradually occurs among electrical adjustment at the time of manufacture, information for correction stored in the device and actual conditions of the device, and the deviation appears as errors in the measured distance.
Further, to perform electrical adjustment or to store information for correction, the facilities for such purposes are needed, and more time is required for the adjustment and this leads to the increase of the manufacturing cost.
To solve the above problems, the present invention provides an electro-optical distance measuring method and an electro-optical distance measuring device, by which it is possible to eliminate the procedures of electrical adjustment on the photodetection circuit in the manufacturing process and to perform distance measurement at higher accuracy to exclude the photodetection circuit, which shows lower phase changes over wide temperature range and without giving consideration on the deterioration of electronic components to the changes over time.    [Patent document 1] JP-A-2004-264116    [Patent Document 2] JP-A-2004-212058    [Patent Document 3] JP-A-2008-82895